1. Field of the Invention
The present invention relates to an electrophotographic toner, an electrophotographic developer, and an image forming method. More specifically, the invention relates to an electrophotographic toner used in an instrument using an electrophotographic method, such as a copying machine, a printer or a facsimile, in particular, a color copying machine; a method for producing the toner; an electrophotographic developer; and an image forming method using the developer.
2. Description of the Related Art
In recent years, the electrophotographic method has widely been used not only in copying machines but also in printers, such as network printers in offices, printers for personal computers and printers for on-demand printing, as information instruments have been developing and communication networks have been making progress in information society. Such characteristics are more strongly requested as high image quality, high speed, high reliability, compactness, lightness, and energy-saving in both fields of monochromic and color electrophotographic processes.
In the electrophotographic method, a fixed image is usually formed through a process comprising: forming an electrostatic latent image on a photoreceptor comprising a photoconductive material by means of various units; using a toner to develop this latent image; transferring the toner image on the photoreceptor, through an intermediate body or without an intermediate body, onto a image receiving body such as a sheet; and then fixing this transferred image onto the image receiving body.
In general, the contact type fixing method, which is widely used as a toner-fixing method, is a method in which heat and pressure are used when a toner image is fixed (hereinafter referred to as the “heating and pressing method”). In the case of this heating and pressing method, the surface of a fixing member and a toner image on a image receiving body contact each other under pressure. Accordingly, the method gives a very high heat efficiency and makes rapid fixation possible. In particular, the method is very useful for high-speed electrophotographic image forming devices.
In recent years, energy-saving performance has been increasingly required. Thus, investigation on low-temperature fixation has been advanced in order to decrease power consumption when a toner is fixed. As a result, several documents report toners comprising a crystalline resin as a binder resin. For example, Japanese Patent Application Laid-Open (JP-A) Nos. 2002-082485, 2000-352839 and 2001-42568 each report a toner comprising a crystalline polyester resin. However, in the case that a crystalline resin is used as a binder resin, there is caused a problem that the electrification quantity of the toner becomes low so that a sufficient developing performance cannot be obtained.
Into fixing devices, the following control is introduced for energy saving: a control which stops power supply to fixing device during standby period; or a control which maintains the fixing device at a lower temperature than a fixing temperature during standby period. Accordingly, at the time of printing, it is necessary to raise the temperature of the devices to the fixing temperature rapidly. Thus, various modifications are made in order to control the temperature of a fixing device or the temperature distribution thereof (JP-A No. 8-220932).
Further, suggested is a method of using a material having a high thermal conductivity as the surface material of a fixing device in order to lower fixing temperature (JP-A No. 5-210330).
However, in a fixing device which involves rapid temperature-rising, as described above, temperature is raised at a rate of 10 to 20° C./second. Consequently, printing starts before the surface temperature of the fixing device becomes even. For this reason, the fixing device has a broad temperature distribution and the temperature difference between the highest temperature region and the lowest temperature region becomes about 50 to 100° C. However, toner is designed to have a narrow fixable temperature range, which is a temperature range between the lowest fixable temperature of the toner to the hot offset temperature. Thus, no toner having a broad fixable temperature range (a broad fixing latitude) has been obtained. If the surface of a fixing device has a high thermal conductivity, the fixing temperature thereof can be lowered. However, the releasing properties thereof become poor so that the fixing temperature range becomes narrow since fixing devices which are good in both of thermal conductivity and releasing properties have not yet been developed (conventional fixing device surfaces made of fluororesin or silicone resin are poor in thermal conductivity, and fixing device surfaces made of alumina, which has a high thermal conductivity, are poor in releasing properties).
For energy saving, the low-temperature fixing toners including a crystalline resin as a binder resin are effective. However, crystalline-resin-containing toners which have been reported hitherto cannot attain a broad fixable temperature range. Thus, the crystalline-resin-containing toners are unsuitable for forming an image by use of a fixing member having a high thermal conductivity, such image formation requiring a broad fixable temperature range of toners.
Accordingly, a toner which has a sufficient image-forming properties and which can be used to form an image by use of a fixing member having a high thermal conductivity has not yet been obtained.